In November of 2020, Waters signed his second consecutive one-year, two-way contract with the Boston Celtics. The young guard from LSU made his season debut on Christmas Day with one assist in 2:30 minutes of play in a loss to the Nets. Waters appeared in 26 games for Boston with three starts. On Feb. 24, Waters scored 11 points (at the time a career high), plus added five assists, during a loss at Atlanta. The young guard saw his minutes increase in late April and May, when he appeared in eight of Boston's last 10 games, with 10 starts. On Apr. 28, Waters started and posted six points, six assists and two steals over 25 minutes of run in a home win over the Hornets. On May 15, the LSU product came off the bench to deliver 14 points (4-8 FG, 3-3 3Pt, 3-3 FT) and seven assists during a win at Minnesota. One night later, in the last game of the regular season, Waters scored a career-high 17 points, and added five assists and three triples, in a road loss to the Knicks. For the season, the point guard averged 3.8 points, 2.4 assists, 0.6 steals and 0.6 made three-pointers over 9.2 minutes per game.2019
The Celtics picked Waters 51st overall in the second round of the 2019 NBA Draft. He went on to split the 2019-20 season between Boston and the G League's Maine Red Claws via a two-way contract. In 11 games as a Celtic, Waters averaged 3.6 points across 10.8 minutes per game, adding 1.1 rebounds, 1.5 assists and 0.9 steals. He saw more opportunities for playing time in the G League, where he averaged 16.0 points, 7.3 assists and 1.9 steals across 36 games. Those figures earned Waters G League Rookie of the Year and All-NBA G League Second Team honors. The LSU product did a little of everything in his NBA debut Nov. 25 against Sacramento, scoring seven points along with three assists, two boards, a steal and two blocks. On Jan. 6, he set his season high in scoring with eight points against the Wizards. Waters got to start for the first time in his career in the final game of seeding play in the Orlando bubble, facing the Wizards again on Aug. 13, and the rookie responded with season highs in both steals (five) and assists (four) along with seven points. He went on to make his NBA playoff debut in Game 5 of the Celtics' second-round series against Toronto, picking up a rebound and an assist in 2:21 on the court.